Molecular-level Simulations of Cellulose Dissolution by Steam and SC-CO2 Explosion

Doktorsavhandling, 2014

Dissolution of cellulose is an important but difficult step in biofuel production from
lignocellulosic materials. Steam and supercritical carbon dioxide (SC-CO2) explosion are two
effective methods for dissolution of some lignocellulosic materials. Loading and explosion
are the major processes of these methods. Studies of these processes were performed using
grand canonical Monte Carlo and molecular dynamics simulations at different pressure/
temperature conditions on the crystalline structure of cellulose. The COMPASS force field
was used for both methods.
The validity of the COMPASS force field for these calculations was confirmed by comparing
the energies and structures obtained from this force field with first principles calculations.
The structures that were studied are cellobiose (the repeat unit of cellulose), water–cellobiose,
water-cellobiose pair and CO2-cellobiose pair systems. The first principles methods were
preliminary based on B3LYP density functional theory with and without dispersion
correction.
A larger disruption of the cellulose crystal structure was seen during loading than that during
the explosion process. This was seen by an increased separation of the cellulose chains from
the centre of mass of the crystal during the initial stages of the loading, especially for chains
in the outer shell of the crystalline structure. The ends of the cellulose crystal showed larger
disruption than the central core; this leads to increasing susceptibility to enzymatic attack in
these end regions. There was also change from the syn to the anti torsion angle conformations
during steam explosion, especially for chains in the outer cellulose shell. Increasing the
temperature increased the disruption of the crystalline structure during loading and explosion.